This invention relates to an airship provided with solar cell array means and to a method of orientating inflight solar array means mounted on an airship.
Since an airship uses only a small amount of energy for propulsion, it is inherently suitable to be powered by solar energy derived from arrays of solar cells mounted on the airship. For many airship applications, including very long endurance unmanned roles, it can be postulated that solar energy is the only practical energy medium acceptable under current ecological agreements for powering such airships.
It is a property of a solar cell that it has maximum efficiently when the incident solar irradiation is normal, i.e. at 90xc2x0, to the cell. In order to capture sufficient energy for powering purposes, it is necessary to mount a large number of solar cells on the airship, for example mounted as a number of arrays secured to the outside body of the airship. Since, for aerodynamic reasons, an airship is usually shaped in the form of body of revolution, the different solar cells will face in different directions at any one time and thus only some of the solar cells will receive the maximum amount of incident solar radiation at any one time. Thus with large arrays of solar cells on the surface of an airship, many solar cells will receive less than maximum energy. Indeed in certain conditions, any solar cell on xe2x80x9cthe dark sidexe2x80x9d of the airship facing away from the sun will receive virtually no solar radiation, except a small quantity of reflected solar radiation.
Furthermore, since any airborne airship must be assumed capable of flying a random course, it cannot be guaranteed that solar radiation will occur over a predictable range of azimuth angle.
This leads to a situation where, up to the present time, airship designers have found it necessary, in order to be able to provide sufficient power to operate an airship, to cover the majority of the airship hull surface with solar cells to ensure that sufficient power can be generated whichever way the airship is directed and wherever the sun is positioned relative to the airship. In practice, when an airship is positioned to receive solar radiation, typically less than half the total number of solar cells of a conventionally arranged cell array mounted on the external surface of the airship collect energy at any one time. This is clearly a highly inefficient system both in terms of cost but, more importantly, in the weight of large solar cell arrays. At high operating altitudes weight is absolutely critical and any weight savings can be crucial in achieving a xe2x80x9cbuoyantxe2x80x9d airship.
There have been many proposed solar powered airships, and a good deal of published work. Most authors accept the limitations of latitude and weather conditions which mitigate against practical use of solar airships. So far as is known, no solar powered airship has succeeded.
A known solar powered aircraft is known from U.S. Pat. No. 5,518,205. This known aircraft has a pair of inflated hulls which are connected by forward and rear wings and from which a gondola is supported by suspension wires. The forward wing carries a solar cell array on its upper surface. By adjusting the position of the gondola the aircraft can be made to bank to optimize exposure of the solar cell array to incident solar radiation. The aerodynamics of such an aircraft are of course considerably altered by the banking of such a twin-hulled aircraft.
It is an aim of the present invention to reduce the number of solar cells required on an airship for the power requirements, e.g. providing propulsion, of the airship.
It is another aim of the present invention to provide means for changing the orientation of solar cell array means mounted on the hull of an airship in order to increase, and preferably to maximize, the amount of solar radiation collected by the solar cell array means.
According to one aspect of the present invention, there is provided an airship comprising gas envelope means for providing in use a hull with a generally longitudinally extending longitudinal axis and solar cell array means arranged in use to be mounted on the outside of the hull, characterized in that the solar array means are arranged in use to extend around only a part of the circumference of the hull, and in that means are provided for turning, in use, the solar cell array means about the longitudinal hull axis to increase the amount of incident solar radiation collected by the solar array means.
By providing means for turning the solar array means about the longitudinal hull axis, the solar array means are able to xe2x80x9ctrackxe2x80x9d the sun as it moves across the sky. For example, the solar array means will be turnable through an arc, about the longitudinal hull axis, between end positions offset on opposite sides of the centre of buoyancy of the airship.
The solar cell array means is preferably rigidly attached to the airship hull, for example, to an upper quarter of the airship hull. In this case the turning means is operable to cause the entire hull to roll to enable the solar array means to track the sun as it traverses across the sky. Although it is preferred for the solar array means to be rigidly attached to the airship hull, the present invention is not so limited and is intended to cover solar cell array means movable relative to the airship hull.
The solar array means extends along a sufficient length of the hull to provide the required area of array for the basic power requirements of the airship.
Preferably the turning means for effecting turning of the solar cell array means includes movable weight means, the centres of gravity of the solar cell array means and the weight means being positioned on opposite sides of the centre of buoyancy of the airship. Thus the weight of the solar cell array means and of the weight means counteract or counterbalance each other and prevent a permanent rolling force acting on the airship. By changing the xe2x80x9clateral offsetxe2x80x9d of the weight means (i.e. the distance between the centre of gravity of the weight means and a vertical plane passing through the centre of buoyancy of the airship), the airship is caused to roll thereby also turning the solar array means fixedly mounted to the airship hull.
In order to maximize weight saving, the weight means preferably comprises a payload module of the airship which can be laterally offset to counter the weight of the array. Since the airship is designed to carry a payload, no additional weight means are required to off-set the weight of the solar array means.
In certain high altitude designs, the payload module may be completely contained within the hull. Thus the airship can be configured in a pendulum stable manner for a particular pointing direction of the solar array means. The payload module is conveniently mounted for movement in an arc about the centre of buoyancy. Thus movement of the payload module along the arc causes the airship to roll about its longitudinal axis until the centre of gravity of the complete vehicle is once again directly below the centre of buoyancy. By moving the payload module in an arc below the centre of buoyancy, pendulum stability can be maintained. Using this system it is possible to achieve turning of the solar array means of in-excess of 90 degrees. This allows the airship not only to align the solar array means as the sun moves through the sky over the daylight hours, but also to allow for the airship to fly in the opposite direction (for prevailing wind and station keeping requirements to be met) and still maintain the solar array means on the sun facing side of the airship. This is particularly important at the higher latitudes of operation.
Conveniently the turning means includes cable and winch means. Typically the cable and winch means comprises a series of small winches pulling in/playing out cables that make up suspension means for suspending the payload module from the airship hull. The control of operating the cable and winch means would form an integral part of the overall flight control system of the airship.
According to another aspect of the present invention there is provided a method of orientating in-flight solar array means mounted externally on a hull of an airship to increase the amount of incident solar radiation collected by the solar cell array means.